Documentation: theme the docs

[cogat]

http://docs.readthedocs.io/en/latest/theme.html

Apparently it only works with Sphinx mode, so we need to switch to that first - and we need a better reason than this. Currently we're using Markdown mode. (You can use both in the same project http://docs.readthedocs.io/en/latest/getting_started.html#in-markdown )

[mrmachine]

Sphinx/rST has much much better support for cross-document links (especially to API docs) and annotations. It's just slightly more fiddly to write, but its the default/defacto for docstrings and Python in general anyway, so we should probably just do it already.

[cogat]

Agreed, but #13 is the reason to use sphinx, not to get a theme :)